LAARS Heating Systems
Page 68
Figure 42. Lead Lag Operation, 2 boilers.
Low demand -
The first boiler in
sequence fires at
less than 65%
First
boiler
Second
boiler
Demand increases -
Once the first boiler
reaches 65%,
the second boiler
switches on, and by
default, both modulate
together between 20%
and 65% (this can be adjusted up to 20% and
85
%).
Nearing max. demand -
Once both reach 65%,
then they are both
allowed to go over
65%
9.H.18 Lag (Slave)
Configuration
About Lead Lag Operation -
Units = Boilers or Water Heaters.
If an installation includes two or more units, they
may be set up for “Lead Lag” operation. One unit will
be set up as the “Lead”, and the others will oper-
ate as “Lags.” Figure 43 shows an installation with
eight units (A Lead Lag system can include up to
eight units.) The units controllers are connected in a
“daisy chain” using a Modbus connection.
NOTE: Lead & Lag within the touchscreen display is
referred to Master & Slave. These are industry terms
and are used interchangably by different control
system manufacturers.
A single System sensor is used to monitor the
demand for heat. The input from this sensor is used
by the Master controller to control the modulation
rates of all of the operating units.
Lead Lag Modulation Cycle -
Note - We will explain the modulation cycle here,
in case you need to understand how the Lead Lag
system actually operates. If you are installing the
unit(s) and want to skip this section, just remember
that, as the heating demand increases, the Lead
Lag system puts more units on-line. As the heating
demand is reduced, the Lead Lag system shuts off
some of the units.
The signal from the System sensor is sent to the unit
operating as the Lead. A Run sequence is initiated
when the system temperature falls to the Lead Lag
Central Heat setpoint. (Actually this is the LL CH
setpoint less the On Hysteresis value. The default
setting for On Hysteresis is 5°F, but this is adjustable.)
Notice that, when a unit is operating as part of a Lead
Lag system, it does not use the normal CH setpoint –
it uses the special LL CH setpoint on the Lead unit.
The unit acting as the Lead decides which
unitisassignedtostartfirst.
This assignment is
rotated across all of the available units so that
Figure 43. “ Lead Lag” Operation in a System with Eight Units.
Note: The Displays on Units 2 thru 8
will display information pertaining only to that specific unit.
Touch
Screen
Lead Lag
Master
Addr
1
Addr
2
Addr
3
Boiler 4
Addr
4
Boiler 5
Addr
5
Boiler 6
Addr
6
Boiler 7
Addr
7
Boiler 8
Addr
8
Boiler 3
Boiler 2
Boiler 1
anyoneunitdoesnotrunsignificantlylonger
than the others. The Lead Lag controller tracks
the run times for all of the units, and uses this
to calculate the starting order for the units. This
means that each time the system starts up, a
differentunitmaystartfirst.
CAUTION
You should set the Modbus addresses before
you connect the Modbus wiring. If the wiring
is attached before the Modbus addresses on
the controls are changed, there will be multiple
controls with the same address, and the system
will not work.
WARNING
